1. Field of the Invention
The present invention relates to light-emitting devices and electronic devices including layers containing organic compounds as light-emitting layers.
2. Description of the Related Art
Light-emitting elements including organic compounds as luminous bodies, which have features of thinness, lightness, high-speed response to input signals, and DC drive at low power supply voltage, have been expected to be applied to next-generation flat panel displays or lighting devices. In particular, display devices in which light-emitting elements are arranged in matrix have been considered to have advantages of a wide viewing angle and high visibility over conventional liquid crystal display devices.
The emission mechanism of a light-emitting element including an organic compound as a luminous body is as follows. First, when voltage is applied between a pair of electrodes with an electroluminescence (hereinafter also referred to as EL) layer provided therebetween, electrons injected from a cathode and holes injected from an anode are injected into the EL layer, so that current flows. Then, the injected electrons and holes bring the light-emitting organic compound included in the EL layer into an excited state, so that light is emitted from the excited light-emitting organic compound. Singlet excitation and triplet excitation are known as excited states, and light can be emitted through either state.
An EL layer included in a light-emitting element includes at least a light-emitting layer. The EL layer can further have a layered structure including a hole injection layer, a hole transport layer, an electron transport layer, an electron injection layer, and/or the like, in addition to the light-emitting layer.
In the case where a device for displaying full-color images is manufactured, it is necessary to arrange light-emitting elements which emit lights of at least three colors of red, green, and blue in matrix. As a method for obtaining lights of at least three colors of red, green, and blue, there are a method by which necessary portions of EL layers are colored separately and light-emitting elements emitting lights of different colors are provided (hereinafter such a method is referred to as a separate coloring method), a method by which each color is obtained by formation of all the light-emitting elements that emit white light and transmission of the white light through a color filter (hereinafter such a method is referred to as a color filter method), a method by which each color is obtained by formation of all the light-emitting elements that emit blue light or light of a color with a shorter wavelength than that of blue and transmission of the blue light through a color conversion layer (hereinafter such a method is referred to as a color conversion method), and the like. For example, Reference 1 discloses an organic EL display device with a color filter method.
The separate coloring method has many technical problems such as low material use efficiency and difficulty in the increase in yields: thus, it is difficult to increase definition or the size of a substrate. In contrast, the color filter method and the color conversion method facilitate manufacturing steps as compared to the separate coloring method; thus, it is easy to increase definition or the size of a substrate.